This application relates generally to magnetic recording and more specifically to a technique for preventing a data loss as a result of deletion of data on adjacent tracks that may occur when data on a given track is written repeatedly in a magnetic disk device.
In recent years, as magnetic disk devices achieve widespread use not only in the field of computer technology but also in another application such as hard disk recorders in place of video tape recorders, the need to increase capacity of the magnetic disk devices, which are random-accessible large-capacity storage units, grows increasingly.
FIG. 2 shows a mechanism of a typical magnetic disk device. The disk device comprises: one or more magnetic disks 201, each of which consists of a disk of a non-magnetic material such as glass on which a magnetic layer is laminated; write heads for writing data to the magnetic disks; and read heads for reading data from the magnetic disks. Typically, pairs of one write head and one read head constitute an integrated magnetic head construction 202. The magnetic disks 201 are held on a single spindle 203. The heads 202 whose number corresponds to the number of surfaces of the magnetic disks are attached to arms 204. The arms 204 are configured so that they can be moved onto the disk surfaces by a VCM (voice coil motor) 205.
In the magnetic disk device, data is written and read on areas disposed on the disk as concentric circles, which are called tracks. FIG. 3 shows the placement of the tracks 302 on the magnetic disk 301. The tracks 302 are disposed so that they are typically spaced uniformly from each other by a track pitch 303 and each track 302 consists of servo areas where information that is needed for positioning the heads and data areas to/from which a user can write/read data. Further, each data area can be divided into minimum accessible units called sectors.
FIG. 4 shows basic components required for writing/reading data on/from the magnetic disk device. When the data is written, the data is sent from a host computer 401 to a hard disk controller (HDC) 402. The HDC 402 determines an address or the head, track and sector number to which the data is written and issues a command to a servo controller 404 to move a write head 407 to a specified sector position. Based on servo information that is written on the magnetic disk 409 and obtained via a R/W amplifier 406 and a R/W channel 405, the servo controller 404 moves the write head to the track in which the desired sector resides.
The HDC 402 outputs the write data to the R/W channel 405 in synchronization with the sector specified on the track of the rotating magnetic disk 409. The write data is encoded into a format suitable for writing in the R/W channel 405 and the R/W amplifier 406 and, then, the encoded write date is written on the magnetic disk 409 by a write head 407. Here, it is to be noted that the data from the host computer is typically stored in a data buffer 403 once and, then, sent from the data buffer 403 to the R/W channel when the writing is ready.
Also when the data is read, the head is positioned on the track where the desired sector resides in a manner similar to the one when the data is written. After the head has been positioned, the data is read from the magnetic disk 409 by a read head 408 in synchronization with the specified sector, the read waveform is decoded into the original data by the R/W amplifier 406 and the R/W channel 405 and, then, the decoded data is sent to the HDC 402. Finally, the HDC 402 outputs the data to the host computer.
The data is written or read in the procedure described above. The addresses specified by the host computer when it accesses the magnetic disk device are called logical addresses, which are not always the same as physical addresses or actual addresses on the disk. A MPU 410 calculates the corresponding physical addresses from the logical addresses specified by the host computer 401 and the actual write/read operation is performed on the addresses. When the magnetic disk device is accessed sequentially, the data is written or read in the order of logical addresses.
In order to meet the requirement for increasing the storage capacity in the magnetic disk device configured as described above, various attempts are made to improve the recording density such as by increasing a track recording density of the disk, which is a density in the circumferential direction, or by reducing the track width as well as the track pitch to increase the track density.
FIG. 5 shows a structural schematic view of the write head. When a coil 503 is energized, a magnetic field is generated between a floating upper surface portion 504 of a head of an upper pole piece 502 and a lower pole piece 501 and, then, as this magnetic field magnetizes the magnetic disk surface, the data is written. However, as the track width is made narrower so as to increase the recording density as described above, the tip portion of the write head becomes narrower, and the tip portion may be saturated by the magnetic field. As a result, the magnetic field may not only be generated at the floating upper surface portion 504 as it should be, but may also from side surfaces 505. Further, when the track pitch is narrow, this leakage magnetic field from the side surfaces may be spread to adjacent tracks. Since the leakage magnetic field is weaker than the main write magnetic field to write the data, even if the leakage magnetic field is spread to the adjacent tracks, the data on the adjacent tracks may not be affected immediately. However, as the adjacent tracks are exposed to the leakage magnetic field multiple times, the data on the adjacent tracks may be deleted little by little and, eventually, the data may become unreadable.
In order to avoid the data loss on the adjacent tracks due to the leakage magnetic field, examples of possible measures include:                (1) increasing the coercive force of the magnetic disk so that the data is not liable to be deleted even if there is the leakage magnetic field from the adjacent tracks;        (2) configuring the write head such that the leakage magnetic field is not liable to be generated;        (3) reducing the amount of the leakage magnetic field to which the adjacent tracks are exposed by increasing the track pitch; and        (4) reducing the amount of the leakage magnetic field itself by adjusting the magnitude of the current applied to the write head when the data is written or an amount of overshoot of the write current waveform.        
Further, in a prior art example, under the circumstance where it is required to increase the track density TPI so that the recording density of the recording media can be improved and, due to the high TPI, both new and old versions of the write data may coexist together in a unitary storage area, an improvement has been proposed wherein incorrect data that may result from reading the old data can be inhibited (for example, see Japanese Patent Laid-open No 2001-338468).
Still further, in another prior art example, paying attention to the high correlation of a failure frequency of storage units with the number of accesses and total energizing time, a technique has been proposed for storing an operation history of a main storage unit every time the main storage unit is operated and, based on this operation history, determining the possibility that the failure may occur in the main storage unit (for example, see Japanese Patent Laid-open No. 2001-350596).
Although the methods for inhibiting the data loss in the adjacent tracks due to the leakage magnetic field have been proposed as described above, if the coercive force of the disk is increased as discussed in item (1), the data on the adjacent tracks becomes less liable to be deleted but it also becomes difficult to overwrite the data as it should be and thus the overwrite characteristic is degraded, which may result in a poor error rate of the data that should be overwritten. On the other hand, with regard to item (2), the construction of the write head that is effective in inhibiting the leakage magnetic field has not been sufficiently apparent up to the present and this problem should be addressed in the future.
Further, if the measure described in item (3) is adopted, in order to ensure the storage capacity per disk, the track recording density must be increased in proportion to the track pitch, but the higher track recording density may result in reduced resolution and S/N ratio of the readout waveform, thereby increasing the error rate. Still further, with regard to the setting of the write current value and the adjustment of the overshoot of the write current as discussed in item (4), if such measures are taken so that the leakage magnetic field will not occur or, more specifically, if the write current value is set to a smaller value or the amount of overshoot is reduced, the data itself may be written insufficiently and, consequently, the error rate may be increased just as in the case described with regard to item (1).
Still further, although the improvement measures against the failure of the data written on the storage media have been proposed in Japanese Patent Laid-open No 2001-338468 and Japanese Patent Laid-open No. 2001-350596 as described above, these measures do not address the data loss on the adjacent tracks due to the leakage magnetic field.